The level of hardness in water can have a deleterious effect in many systems. For example, when hard water is used in conjunction with cleaning compositions, hard water can cause precipitation of hard water scale or components of a cleaning agent. In general, hard water refers to water having a level of calcium and magnesium ions in excess of about 100 ppm. Often, the molar ratio of calcium to magnesium in hard water is about 2:1 or about 3:1. Although most locations have hard water, water hardness tends to vary from one location to another.
Hard water can cause stains on articles as a result of a visible film depositing onto the surface of the articles. The film may be caused by calcium present in hard water precipitating and depositing onto the surface. To prevent such precipitation, cleaning compositions can include a threshold agent. A threshold inhibitor or threshold agent inhibits crystallization of water hardness ions (e.g., calcium containing salts) from solution, but does not necessarily form a specific complex with the water hardness ion.
In some circumstances, precipitation of calcium salts can be beneficial. Etching or corrosion of glass or aluminum is a common problem in warewashing and surface cleaning. Glassware that is repetitively washed in automatic dishwashing machines has a tendency to develop a surface cloudiness that is irreversible. The cloudiness can manifest itself as an iridescent film that displays rainbow hues in light reflected from the glass surface. The glass becomes progressively more opaque with repeated washings. It is believed that the glassware corrosion problem relates to two separate phenomena; the first is corrosion or etching due to the leaching out of minerals from the glass composition itself together with hydrolysis of the silicate network, and the second is deposition and redeposition of silicate material onto the glass. Common corrosion inhibitors work by causing controlled precipitation of calcium salts, which can reduce such etching or corrosion.
Thus, it is counterintuitive to include a threshold agent, which normally prevents precipitation of calcium salts, in a corrosion inhibitor. Further, it is entirely unexpected that a mixture of hardness ions (e.g., magnesium ions and calcium ions) in an amount greater than the amount of threshold agent would have a beneficial effect on etch or corrosion.